This invention relates to a multiple input signal high-speed analog-digital converter circuit for converting a number of analog signals into digital signals at high speed.
A multiple input signal analog-digital converter circuit finds use in devices such as a combinatorial weighing apparatus.
A combinatorial weighing apparatus operates by supplying a plurality of weighing machines with articles to be weighed, computing combinations based on weight values obtained from the weighing machines, selecting a combination giving a total combined weight value equal or closest to a target weight, and discharging the articles solely from those weighing machines corresponding to the selected combination, thereby providing a batch of weighed articles having a weight equal or closest to the target weight.
Such a combinatorial weighing apparatus will now be described briefly with reference to FIG. 1. The apparatus includes load cells 10.sub.-1, 10.sub.-2 . . . 10.sub.-n for sensing the weight of articles introduced into the respective weighing hoppers belonging to n weighing machines. Each load cell produces a weight signal, namely an analog value indicative of the weight sensed thereby. The weight signals from these load cells 10.sub.-1, 10.sub.-2 . . . 10.sub.-n are applied as multiple input signals S.sub.1, S.sub.2 . . . S.sub.n to a multiplexer 13 via amplifier circuits 11.sub.-1, 11.sub.-2 . . . 11.sub.-n and filters 12.sub.-1, 12.sub.-2 . . . 12.sub.-n, respectively. The multiplexer 13, which is composed of analog switches or the like, responds to a selection signal S.sub.cl from a computation controller 20, described below, by selectively applying the weight signals S.sub.1, S.sub.2 . . . S.sub.n as a weight data signal S.sub.o to a buffer circuit 14 sequentially in a time series. The buffer circuit 14 delivers the weight data signal S.sub.o received from the multiplexer 13 to a sample/hold circuit 15 upon subjecting the signal to an impendance conversion. The sample/hold circuit 15 repeatedly samples and holds the weight data signal S.sub.o subjected to the impedance conversion by the buffer circuit 14, and delivers the weight data signal to a buffer circuit 17. The latter subjects the signal to an impendance conversion, producing an analog weight data signal S.sub.p which is delivered to an analog-digital converter (A/D converter) 18. The latter digitizes the analog weight data signal S.sub.p to produce a digital output S.sub.d which is applied to a computation controller 20. The latter is composed of a microcomputer and includes a processor (CPU) 21 for performing combinatorial processing, a read-only memory (ROM) 22 for storing a processing program for combinatorial processing, and a random-access memory (RAM) 23 for storing the weight data as well as the results of processing performed by the processor 21. The computation controller 20 computes combinations on the basis of the weight data, selects a combination giving a total combined weight value equal or closest to a target weight, and delivers a drive signal to drive units 25.sub.-1, 25.sub.-2 . . . 25.sub.-n of respective weighing hoppers belonging to those weighing machines which correspond to the selected combination.
Thus, the multiple input signal A/D converter circuit converts, into a digital value, the analog weight data signal S.sub.o delivered successively in a time series by the multiplexer 13. As shown in FIG. 2, the A/D converter circuit successively samples and holds the weight data S.sub.1, S.sub.2, S.sub.3 . . . S.sub.n by means of the single sample/hold circuit 15 before delivering the data to the A/D converter 18. Unless the sample/hold circuit 15 operates at a sufficiently long sampling time, a capacitor 16 (FIG. 1) for raising sampling precision cannot be charged. Accordingly, the A/D converter 18 is inoperative during sampling time, which is on the order of about 10 usec. This means that the conventional multiple input signal A/D converter circuit cannot make effective use of the A/D converter 18 despite the fact that the latter is capable of performing a conversion at high-speed.